A soft and vibration-free positioning controller for series elastic tendon-driven robots by using gyro sensor and electromagnetic brakes

In this paper, we propose a series elastic tendon-drive controller for soft physical human-robot interaction. While the tendon elasticity enables a safe task execution under hand contact conditions, it poses overshoot and vibration problems in tasks without hand contact. In our implementation, we install an electromagnetic brake in the pulley at the middle of the cable path, to stabilize the vibration. This method enables a wide range of viscosity changes, because the elements are arranged so that the brake force is directly transmitted to the arm. The elastic tendon and brake connection can be modeled as a parallel viscoelastic element. Viscous friction is dominant when the brake is ON, and elastic force is dominant when the brake is OFF. We propose a sliding mode controller by using the electromagnetic brake. High-speed friction control by the electromagnetic brake allows a vibration-free positioning controller. Unlike conventional sliding mode controllers, softness (low stiffness) of the controlled arm is not lost because high-gain position feedback is not used. Stability condition of the sliding mode is expressed in terms of the equilibrium and target positions of the arm angles. The proposed controller was tested on a one-link prototype arm, where a natural rubber tubing is used as the elastic string element, and the tendon is driven by a 3D-printed winding drum. We demonstrated the natural motion of the arm in viscous and elastic modes in the phase plane trajectories and time responses. Then, the results of positioning control by the proposed sliding mode control were demonstrated.